In the morning, Marco sold 12 cups of lemonade for $3. By the end of the day, he had earned $9. How many cups of lemonade did he sell in all? Kaycee wrote the proportion for this situation. Identify the error and give two ways to write the proportion correctly.

Answer:

Kaycee did not keep the units (cups and dollars) in the same positions in both ratios. In her proportion (12/3 = 9/c), the first ratio is cups/dollars, while the second ratio is dollars/cups. Two ways to write the proportion correctly are 12/3 = c/9 (keeping cups in the numerators) or 3/12 = 9/c (keeping dollars in the numerators).